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Allergic rhinitis is very common, affecting approximately 16% of the U.S. population (1). Although therapy with intranasal corticosteroids and intranasal cromolyn has gained acceptance in recent years (2), antihistamines (H1-receptor antagonists) remain the first-line treatment. The possible differential effects of sedating compared with nonsedating antihistamines on driving performance and the risk for motor vehicle crashes represent an important consideration in choosing between the two categories of agents in patients who drive.

In this issue, Weiler and colleagues (3) report on a study in which they used a driving simulator to compare the effects of various agents on measures of driving performance: 60 mg of fexofenadine (a nonsedating antihistamine), 50 mg of diphenhydramine (the prototypic sedating antihistamine), alcohol (sufficient to produce an estimated 0.1% blood alcohol concentration, which is greater than or equal to the legal driving limit in all states with a specified limit), and placebo. This randomized, double-blind, single-dose study used as its primary outcome a measure known as coherence, the degree to which the study participant matched the speed of the vehicle that he or she was following. The investigators found that the mean coherence score after participants took diphenhydramine (0.877) was statistically significantly lower (that is, worse) than the mean score after they took alcohol (0.920), fexofenadine (0.915), or placebo (0.906). In most but not all secondary end points, scores for both diphenhydramine and alcohol were statistically worse than those for placebo but fexofenadine and placebo did not differ significantly. Weiler and colleagues also found that subjective feelings of drowsiness did not correlate well with impairment, suggesting that patients receiving antihistamines may not be able to judge when they are impaired.